1. Field of the Invention
The present invention relates to an analog-to-digital converter (ADC), and in particular, to an ADC for converting an optical signal into an electrical signal.
2. Description of Related Art
FIG. 1 is a circuit block diagram of a conventional analog-to-digital converter (ADC). Referring to FIG. 1, the conventional ADC 10 includes two comparators 101 and 102, a control logic unit 103, a counter 104, a capacitor 105, a resistor 106, and two switch units 108 and 107. One terminal of the switch unit 107 is selectively coupled to an input voltage Vin or a reference voltage Vref, and the other terminal thereof is coupled to the negative input terminal of the comparator 101 through the resistor 106. The positive input terminal of the comparator 101 is grounded, and the output terminal thereof is coupled to the negative input terminal of the comparator 101 through the capacitor 105. The switch unit 108 spans over the capacitor 105, and the capacitor 105 is short-circuited when the switch unit 108 is turned on.
Additionally, the output terminal of the comparator 101 is coupled to the negative input terminal of the comparator 102. The positive input terminal of the comparator 102 is grounded, and the output terminal thereof is coupled to the control logic unit 103. Accordingly, the control logic unit 103 can control the switch unit 107 to couple the negative input terminal of the comparator 101 to the input voltage Vin or the reference voltage Vref and control the on/off of the switch unit 108 according to the output of the comparator 102 and a clock signal CLK. In addition, the output terminal of the control logic unit 103 is coupled to the counter 104 so that the counter 104 counts the clock signal CLK according to the output of the control logic unit 103 and generate a digital signal.
In general, the comparators 101 and 102 are respectively composed of a plurality of transistors. However, if these transistors are fabricated through low temperature poly-silicon processes, the threshold voltages of the transistors will drift along the operation time thereof. Incorrect digital signal may be output by the counter 104 if the threshold voltages of the comparators 101 and 102 drift.
To resolve foregoing problem and optimize the conversion from an analog signal to a digital signal, an ADC with reduced transistor threshold voltage drift is provided by the present invention.